1. Technical Field
The present disclosure relates to a class-D amplifier, in particular to a modulation method for a switching modulator of a class-D amplifier.
2. Description of Related Art
A class-D amplifier or switching amplifier is an electronic amplifier in which the amplifying devices (transistors, usually MOSFETs) operate as electronic switches, instead of as linear gain devices as in other amplifiers. The signal to be amplified is a train of constant amplitude pulses, so the active devices switch rapidly back and forth between a fully conductive and nonconductive state. The analog signal to be amplified is converted to a series of pulses by pulse width modulation, pulse density modulation or other method before being applied to the class-D amplifier. After amplification, the output pulse train can be converted back to an analog signal by passing through a passive low pass filter consisting of inductors and capacitors. The major advantage of a class-D amplifier is that it can be more efficient than analog amplifiers, with less power dissipated as heat in the active devices.
Referring to FIG. 1, FIG. 1 is a circuit diagram of a switching modulator in a typical class-D amplifier. The class-D amplifier 1 comprises a switching modulator formed by switches 12, 14, 16, and 18 as an output stage. A supply voltage VDD is applied to first ends of the switches 12 and 16, and a ground voltage VSS is applied to second ends of the switches 14 and 18. Second ends of the switches 12 and 16 are respectively electrically connected to first ends of the switches 14 and 18. The second end of the switch 12 and the first end of switch 14 are electrically connected to a first terminal of a load 10 to output a first output signal A, and the second end of the switch 16 and the first end of the switch 18 are electrically connected to a second terminal of the load 10 to output a second output signal B. Switching signals VAH, VAL, VBH, VBL, are respectively applied to control ends of the switches 12, 14, 16, and 18, wherein the switching signals VAH, VAL, VBH, VBL are generated according to a data signal, such as a pulse width modulation signal.
Referring to FIG. 1 and FIG. 2, FIG. 2 is a waveform diagram of a first and second output signals by using a ternary modulation method for a switching modulator. In the example of FIG. 2, the ideal first output signal A is a data signal presented as a pulse width modulation signal with a width of 20 nano seconds, and the ideal second output signal B is just a ground voltage. The amplitude of the first output signal A is PVDD. Therefore, in the ternary modulation method, an area of the differential signal between the first output signal A and the second output signal B is 20*PVDD in the ideal case.
However, in reality, a rising time and a falling time of a first output signal A are not equal to each other, and a rising time and a falling time of a second output signal B are not equal to each other, either. In the example of FIG. 2, the rising time of the first output signal A is assumed to be 6 nano seconds, the falling time of the first output signal A is assumed to be 3 nano seconds, and thus the first output signal A has the amplitude of PVDD in a width of 14 nano seconds. Therefore, the area of the differential signal between the first output signal A and the second output signal B is 18.5*PVDD in the actual case. Accordingly, substantial harmonic distortion is introduced, and performance of the class-D amplifier is bad.
Referring to FIG. 1 and FIG. 3, FIG. 3 is a waveform diagram of a first and second output signals by using a quaternary modulation method for a switching modulator. In the example of FIG. 3, the ideal first output signal A is a data signal presented as a pulse width modulation signal with a width of 40 nano seconds, and the ideal second output signal B is a square signal with a width of 20 nano seconds. To put it concretely, originally, the data signal presented as an pulse width modulation signal has a width of 20 nano seconds, the second output signal B is just a ground voltage, and then, the quaternary modulation method extends two edge sides of the original pulse width modulation signal with 10 nano seconds, and adjusts the second output signal B to be the square signal with the width of 20 nano seconds. The amplitudes the first output signal A and the second output signal B are PVDD. Therefore, in the quaternary modulation method, an area of the differential signal between the first output signal A and the second output signal B is 20*PVDD in the ideal case.
As mentioned above, in reality, a rising time and a falling time of a first output signal A are not equal to each other, and a rising time and a falling time of a second output signal B are not equal to each other, either. In the example of FIG. 3, the rising time of the first output signal A is assumed to be 6 nano seconds, the falling time of the first output signal A is assumed to be 3 nano seconds, thus the first output signal A has the amplitude of PVDD in a width of 34 nano seconds, and the second thus the second output signal B has the amplitude of PVDD in a width of 14 nano seconds. Therefore, the area of the differential signal between the first output signal A and the second output signal B is 20*PVDD in the actual case. The area of the differential signal in the actual case is the same as that in the ideal case, and harmonic distortion is reduced, and performance of the class-D amplifier is improved. Since the second output signal is adjusted to be the square signal with the width of 20 nano second, the quaternary modulation method still has issue of higher inrush current when the class-D amplifier is powered on.